I figure I'll make a thread about this because I've been through so many now - one being the dumb young ST who doesn't have access to his emotions, and several as the close friend or relative confidant - both men and women, hetero and non hetero. I've heard the break up calls, the pre break up concerns, the aftermath, the tipping points, all of that. If you have any questions, you can state them here. I don't post here much and I'm not particularly concerned about professing expertise, but I feel like since I have such a specific set of information about this I might as well have some place to talk about it.
The most general pattern for delta NFs (that are interested in a monogamous and "serious relationship", let's say), is that certain cues are met that inspire that they can have a version of the relationship they seem acceptably close to the ideal that they want to share with someone else. There's a sort of trial period where there is scrutinizing between the ideal and how things work, how things are resolved, how attention is given, etc. The older (and wiser, IMO) that the NFs get the less time they linger here when they aren't getting what they ultimately need. Different things accelerate that period, depending on the individual's wants. Decisions are made by reaching conclusions about whether or not this person has a chance at meeting the ultimate goal and how much the possibilities are exhausted for doing so.
I don't think that's a particularly surprising or rare pattern for this type among others, but I have a lot of contextual experiences that impact how things play out. Enneagram and instinctual stackings affect a lot on how things are prioritized, or what attention looks like. The biggest problem is the lopsidedness of intra and interpersonal awareness towards the NFs. The most common source for continued or lingering drama is that the NFs have a heard time measuring or evaluating 'truth' or 'reality' when it comes to people's interactions towards themselves. NFs are better off and more confident when they have learned how to evaluate people's treatment of them in clear ways that they can have conviction about, and have confidence at resolving grey areas rather than letting things fester within them.
If you have anything you wonder about or would like to discuss, you're invited to do so.